1. Technical Field
The present invention relates to a data processing system having a server and terminal devices interconnected through a network.
2. Description of Related Art
Conventionally, a system configured such that a client (e.g., a digital copier, a scanner, a multi function peripheral) connected to a LAN (Local Area Network) and a host computer (i.e., a server) are interconnected through a global network (e.g., the Internet). Particularly, there is known a system, wherein the host computer achieves a function that the client does not have instead of the client. An example of such a system is disclosed in Japanese Patent Provisional Publication No. HEI 9-238215.
In a general communication system configured as above, with respect to security, an access from (or through) the Internet to the client side is restricted in order to prevent inappropriate access to the client. For example, the above system is configured such that the LAN and the Internet are connected through a broadband router, from among pieces of data transmitted from the server to the client, ones other than responses to requests transmitted from the client to the server are shielded in accordance with a firewall function of the broadband router. That is, if a user of the server intends to transmit commands or data to the client, the communication may be prevented due to the firewall function of the broadband router.
Therefore, in the above-configured communication system, a command request should be transmitted from the client to the server, and then, as a response to the command request, the server transmits the command/data to the client.
According to such a configuration, however, the server is required to transmit some response to the client, in response to the command request from the client, even if there is no command/data to be transmitted from the server to the client. For example, if a proxy server is provided between the client and the server, the server should reply to the request by the client. If no response is issued by the server within a predetermined timeout period, it is determined that a communication error has occurred even if the server does not have any problem.
Further, if there is a command to be transmitted from the server to the client, it cannot be transmitted until the server receives the command request from the client. Therefore, in comparison with a case where the server can immediately transmit a command, a waiting time period for which the server waits for receipt of the command request from the client should be consumed unnecessarily. Such a waiting period can be shortened by increasing transmission frequency of the command request from the client to the server. However, to increase the transmission frequency results in increase of network traffic, and thus, the frequency cannot be increased unlimitedly.
Alternatively, a firewall setting of the router may be changed in the LAN side so that the command/data can be received through the Internet. However, to change the setting of the router is relatively troublesome, and more importantly, requires the user to change security policy, which may cause a problem.